Novel desiccant

ABSTRACT

A solid methyl cellulose desiccant wherein from about 0.6 to 1.8, preferably 0.9 to 1.5 of the available hydroxyl groups of the anhydroglucose units of the cellulose entity have been replaced by methyl, or methyl and hydroxy alkyl or carboxy alkyl groups, if any, containing from 2 to about 4 carbon atoms, e.g. hydroxy ethyl, hydroxy propyl, carboxyl methyl and the like. In the solid methyl cellulose desiccant, at least one of the substituting groups must be methyl, with the remainder of the substituting groups, if any, hydroxy alkyl or carboxy alkyl, or both; preferably carboxy methyl, if any. At least one-half of the substituting groups are methyl, and preferably at least two-thirds of the substituting groups are methyl; the balance of the substituting groups, if any, being hydroxy alkyl or carboxy alkyl groups. Methyl cellulose of this character can be contacted with a water-containing stream at a temperature below which it will have great affinity for and will absorb significant amounts of water; and conversely above this temperature the methyl cellulose will not only lose this affinity but in addition may give up a large portion of any water it had previously absorbed.

Many industrial processes produce water as a by product of a chemicalreaction, the water as a contaminating constituent of a fluid streamfrom which it must be separated, and removed. In some naturalenvironments too water is a desirable product, and its recovery is aworthwhile objective in itself. For example, in the production ofabsolute alcohol, e.g. ethyl alcohol from a constant boiling mixture ofethyl alcohol and water, it is essential to remove the water from theadmixture to form this useful industrial chemical compound. Illustrativealso, in the production of methyl and ethyl cellulose water is producedas one of several by-products, and water must be separated from thereaction mixture to avoid further hydrolysis reactions which produceadditional undesirable by-products. Desiccants, as is known, can be usedfor the separation of water from such streams, even the separation andrecovery of water from air, but desiccants are not nearly so energyefficient as desired, particularly in todays short energy supplysituation.

Known desiccants include bauxite, inorganic oxide gels, notably silicaand alumina, silica-alumina, activated carbon, molecular sieves and thelike. Desiccants for the dehydration of liquids, e.g. alcohols, are alsowell known and include such materials as sodium sulfate, copper sulfate,zinc chloride, mercuric chloride, potassium carbonate, carboxy methylcellulose and the like. Solids desiccants are generally used in palpableparticulate form and include such shapes as tablets, pellets, spheresand the like. Liquids, e.g. ethylene glycol, have also been used asdesiccants, or absorbents, as for the dehydration of alcohols. In theuse of desiccants for most industrial purposes the dehumidification stepis conducted in a packed column and, when the desiccant is sufficientlywet that its water adsorption capacity is impaired, it is revivified orregenerated by contact with a dry gas, generally at elevatedtemperature. Generally too, the dehydration and revivification steps areconducted in parallel vessels to provide a continuous operation. Thenotoriously energy inefficient portion of the operation resides in therevivification, or regeneration of the desiccants. Large quantities ofheat are required to dry the wet desiccants, and one-step operationswherein the wet desiccant is discarded are generally even moreprohibitive.

It is, accordingly, the primary objective of the present invention toprovide a new and novel composition, or desiccant useful for thedehumidification, or dehydration of wet materials contacted therewith.

A specific objective is to provide a new and novel desiccant,particularly useful for the dehumidification, or dehydration of fluids;notably gaseous, liquid or mixed phase streams.

A further object is to provide a desiccant, or drying agent ascharacterized which can be easily regenerated by low temperature heat.

Yet another, and more specific object is to provide a desiccant, andprocess utilizing said desiccant, which is suitable for the extractionof water from a fluid, notably cool air, and the wet desiccantthereafter heated to a higher, slightly elevated temperature, notably bythe rays of the sun, at which temperature the desiccant will release itsabsorbed water to its surroundings and become dry, as well as lose itsnormal afinity for water at the elevated temperature; and whichdesiccant will regain its normal affinity for water on being broughtback to said lower temperature.

These objects and others are achieved in accordance with the presentinvention which embodies a solid methyl cellulose desiccant wherein fromabout 0.6 to about 1.8, preferably from about 0.9 to about 1.5, of theavailable hydroxyl groups of the anhydroglucose units of the celluloseentity have been replaced by methyl, or methyl and other hydroxy alkylor carboxy alkyl groups, if any, containing from 2 to about 4 carbonatoms, e.g. hydroxy ethyl, hydroxy propyl, methyl hydroxy ethyl, methylhydroxyl propyl, carboxy methyl, carboxy ethyl, carboxy propyl groups,or the like. In the solid methyl cellulose desiccant, at least one ofthe substituting alkyl groups must be methyl, with the remainder of thesubstituting groups, if any, either hydroxy alkyl or carboxy alkyl orboth; preferably carboxy methyl, if any. At least one-half of thesubstituting groups are methyl, and preferably at least two-thirds ofthe substituting groups are methyl; the balance of the substitutinggroups, if any, being hydroxy alkyl or carboxy alkyl groups. Methylcellulose of this character can be contacted with a water-containingstream at a temperature below which it will have great affinity for andwill absorb significant amounts of water; and conversely above thistemperature the methyl cellulose will lose this affinity and not absorbany significant amount of water. This temperature, defined as thesyneresis temperature of the methyl cellulose, can be varied over arange of from about 100° F. to about 195° F., more often over a range offrom about 140° F. to about 160° F., dependent upon the degree ofsubstitution and, to some extent, upon the nature and proportion of thesubstituent groups themselves.

Methyl cellulose, as known, is a cellulose ether comprised of linearchains of β-anhydroglucose rings, manufactured from cellulose.Cellulose, in its natural state is polysaccharide composed of a variablenumber of individual anhydroglucose units linked together through the 1and 4 carbon atoms with a β-glucosidic linkage characterized, e.g., byHaworth et al, as follows: ##STR1## Haworth, W. N., Hirst, E. L., andThomas, H. A., "Polysaccharides, Part VII," J. Chem. Soc. 824(1931). Thehydroxyl groups of cellulose are the primary reaction sites and, it willbe noted, comprise, with the moieties to which they are associated, aprimary alcohol attached to the number 6 carbon atom and secondaryalcohols attached to the number 2 and 3 carbon atoms, these being siteswhich can react to form ethers of cellulose. The hydroxyl groups of amonomer unit (N-2), through the exertion of hydrogen bonding, causeconsiderable intermolecular attraction of the Van der Wall type betweenchains, thus lessening the reactivity normally expected of alcoholsuntil such time as the cellulose has become solvated.

The number of monomer units in a single chain of cellulose can rangefrom a few hundred, or less, to several thousand, e.g., 30 to 5000, ormore, and the structure of cellulose is basically crystalline in nature,partially due to the stiff glucosidic chains, the presence of thehydroxyl groups, and because of the length and flexibility of the chainswhich results in their entanglement. Some amorphous regions, however, doexist in the cellulose chain. Due to the intermolecular forces holdingthe molecules together, solvent penetration is more difficult than inlower molecular weight compounds but various techniques are well knownto the art for hydrolysis or solvation of cellulose, and after solvationthe primary and secondary hydroxyl groups become accessible so thatethers can be formed. Chemically, after solvation of the cellulose,cellulose reacts basically in the same manner as primary and secondaryalcohols. The average number of hydroxyl groups replaced, based on thethree available hydroxyl groups per anhydrogluclose unit, determines thedegree of substitution (D.S.) on the chain. A fully substitutedcellulose derivative, e.g., would have a degree of substitution of 3.0whereas a degree of substitution of 0.6 would mean that an average oftwenty percent of the available hydroxyl reaction sites have beenreplaced while eighty percent remain as free hydroxyl groups; or, adegree of substitution of 1.8 would mean that an average of sixtypercent of the available hydroxyl reaction sites have been replacedwhile forty percent remain as free hydroxyl groups. In accordance withthe present invention, e.g., a solid methyl cellulose desiccant having aD.S. of 0.6 would have an average of twenty percent of the availablehydroxyl reaction sites substituted by methyl, or at least one-half(10%) and preferably two-thirds (16%) of the available hydroxyl reactionsites substituted by methyl, with the balance of the substitutedreaction sites containing hydroxyl alkyl or carboxy alkyl groups. Asolid methyl cellulose having a D.S. of 1.8, on the other hand, wouldhave an average of at least 60 percent of its available hydroxylreaction sites substituted by methyl, or at least one-half (30%), andpreferably at least two-thirds (40%) of the available hydroxyl reactionsites substituted by methyl, with the balance of the substitutedhydroxyl reaction sites containing hydroxyl alkyl or carboxy alkylgroups. Substitution occurs most readily within the amorphous regions ofthe cellulose, and the degree of substitution has marked effect on thesolubility of the of the substituted cellulose; the more substituted thechain, the more soluble the substituted product or derivative up to acertain limiting number above which the nature of the solubility changessuch that very highly substituted products may not be soluble in waterbut may be soluble in organic solvents.

Strong bases can thus be used to partially solvate the naturalcellulose, causing it to swell, thus allowing for easier penetration byreactants for formation of ethers. The swelling or etherifying agentsdisrupt hydrogen bonding and other secondary forces bonding the hydroxylgroups and thereby increases the uniformity of access so that thereactions can be produced. Some common swelling agents are alkali metalhydroxides, e.g., the hydroxides of potassium, lithium, cesium,rubidium, and the like, liquid ammonia, trimethylsulfonium hydroxide,guanidinium hydroxide, cupraammonium hydroxide, trimethylbenzylammoniumhydroxide, and the like. Alkali or alkaline earth metal hydroxides areparticularly preferred and are commonly used to effect partial solvationbecause it is low in cost and provides desired uniform productdistribution with minimum degration.

The presence of the methyl groups in the solid methyl cellulosedesiccant of interest is essential in order to develop a low gelationtemperature, this being essential to develop in the solid methylcellulose desiccant of interest the high affinity for water at lowtemperature. Within the cellulose chain the methyl group is unique inthat is it ideally sized to provide hydrogen-bonding adsorption sites,or "voids" sufficient for the adsorption, and retention of water below agiven syneresis temperature. The presence of some hydroxyl alkyl orcarboxy alkyl groups is permissible, and in some instances desirable,but because of their size, the distance between the cellulose chains isincreased and so, consequently, is the ability of the methyl celluloseto reject the water hydrating the hydroxyl groups to obtain gelation.Moreover, not only does increased substitution per se increase gelationtemperature, but the increased size of the substituting groupsthemselves produces increased gelation temperature which is undesirable.

The solid methyl cellulose desiccant of this invention is thus a uniquespecie within the class of methyl cellulose ethers, and it has uniqueproperties as contrasted with known members of this class which areprepared for use in solution, and have commercial value because of thischaracteristic. The following energy efficient usages thus exemplify thesolid methyl cellulose desiccants of this invention, to wit:

(1) A fermentation broth can be distilled to recover a constant boilingmixture, or azeotrope, containing 95.57 percent ethyl alcohol in water.This mixture contacted with a solid methyl cellulose desiccant (DS=1.8),the substituting groups being all methyl, to preferentially absorb thewater away from the mixture. The wetted solid desiccant can then beseparated from the essentially pure, absolute alcohol, dried in a streamof air, or in the ambient atmosphere in sunlight, and then reused.

(2) A wet product vapor stream from a vessel within which conventionalmethyl cellulose is being prepared can be contacted with the desiccantof (1), supra, and the water preferentially absorbed. Water thus removedfrom the product stream results in the suppression of hydrolysisreactions which normally lead to the production of undesirableby-products, and consequently, to the consumption of costly reactants.

(3) Water can be obtained from cool night air by passage over thedesiccant of (1), supra, packed within a tube, to saturate it withwater. By exposure of the tube of wetted desiccant to the morning sun,the temperature of the desiccant exceeds its syneresis temperature,about 140° F., and the water is released by the desiccant. An air streampassed through the tube can pick up the water, and the air then cooledin a water condenser to recover the water; thus making water availablein the desert using the sun as the only energy source.

The polymeric, solid methyl cellulose desiccant exemplified in (1), (2)and (3), supra, will thus adsorb water very rapidly at temperaturesbelow 120° F. In contrast, when the temperature of the desiccant isincreased to 140° F., the desiccant suddenly and completely loses itsability to adsorb and retain water, but rather releases water to itssurroundings. It is believed that this absorption-desorption phenomenonresults from the ability of the polymer molecules to hydrogen bond toeach other above the syneresis temperature, while below the syneresistemperature the polymer preferentially bonds to water molecules. Theimportance of this phenomenon is that the polymeric, solid methylcellulose of this invention can be dried at very low temperatures,particularly at solar temperatures such as those available during normaldaylight hours. Even though other desiccants are known therefore, hightemperatures are required for drying such desiccants this makingconventional heating, or refrigeration which may use such desiccants, atbest, only solar assisted. The present unique methyl cellulose desiccantmakes possible true solar heating, and refrigeration.

The process of this invention, at the heart of which lies the novelsolid methyl cellulose desiccant, requires generally an initial contactof a cool moisture-containing stream of air with said desiccant at belowits syneresis temperature at which point in time the desiccant willadsorb and retain water, with concurrent adiabatic heating of thedehumidified air. The warm dehumidified air, as would be expected, canbe used to maintain an enclosure, or facility, e.g. residence, plantwork space or office, at comfortable temperature during cold months,e.g. as in winter. In a subsequent step, where refrigeration is desired,as in summer, the desiccant can be used to dry ambient air with theaccompanying generation of heat. The hot dry air is then cooled byexchange with ambient air, and the cool dry air then passed through awater evaporator which further cools down the air to a comfortabletemperature for use in cooling an enclosure, or facility, e.g.,residence, plant work space or office. The desiccant can then be driedby raising its temperature in a solar collector to above the geltemperature.

The FIGURE discloses a house incombination with a ventilation systemincluding the methyl cellulose desiccant.

The process, and the principle of its operation, will be betterunderstood by reference to the following more detailed description of apreferred emboiment, and to the attached FIGURE to which reference ismade as the description unfolds. This embodiment shows methods by meansof which a typical residence can be heated or refrigerated.

Referring to the FIGURE there is depicted a typical residence 10, orhouse with roof 11 within which is mounted one or a plurality of tubes12 packed with the solid methyl cellulose desiccant of this invention,one segment of tubes of which are shown for simplicity of illustration.A pump 13 located at a corner of the house can withdraw ambient, coolair from the atmosphere and pump same via line 14 into a header 15 fromwhich it is introduced through the series of tubes 12 packed with thesolid methyl cellulose desiccant of this invention. The air, on passagethrough the desiccant filled tubes 12 has its water removed, or isdried, while simultaneously the air is warmed. The warm dry air isdischarged from tubes 12 into the header 16, and then into the house 10via line 17 and manifold 18. The warm air is distributed throughout thehouse by ductwork (not shown). The desiccant can be regenerated byreversal of the air flow by withdrawal of air from ambient and feedingsame into line 17 at a period when the house need not be warmed, orwhile another unit (not shown) is supplying warm air to the interior ofthe house.

By precooling the warm dry air, as by heat exchange means not shown,e.g. with ambient air, with subsequent passage of the cool dry air intothe house 10 through a humidifier 18, on the other hand, the cool dryair can be further cooled to refrigerate the house, as in summer whencooling is needed.

Integration of the tubes 12 into conventional solar collection panels(now shown) to permit the rays of the sun to play upon the tubes duringthe period of regeneration provides an energy efficient means toregenerate the desiccant. It is thus but a simple matter to shade thetubes during the adsorption portion of the cycle, and to expose thetubes to the sun during the regeneration portion of the cycle to desorbthe moisture from the desiccant.

Whereas it is known to use a drying agent, or desiccant to initially dryhumid air, which causes adiabatic heating, with subsequent cooling ofthe adiabatically heated air by heat exchange with ambient air, andfinal vaporization of water into the air with some type of humidifier toobtain refrigeration, the difficulty with such processes is that oncethe desiccant or drying agent is saturated with water it has to be driedwith high temperature heat in order to make the desiccant give up itsmoisture. Most systems must therefore burn natural gas, butane or streamto regenerate the drying beds. Quite obviously if high temperature heatmust be used to remove the water from the desiccant, this same heatcould more advantageously by used to run an ammonia evaporationrefrigeration system or conventional cooling, and consequently, presentdesiccants do not provide suitable means to make solar heating, andrefrigeration a reality. Real world economics, today requires a lesseneduse of, or avoidance of fossil fuels as energy sources for low costheating, and regrigeration.

In the process of this invention, on passage of air over the solidmethyl cellulose derivative (D=1.8), the desiccant, for example, pickedup 17% moisture when exposed to 80% relative humidity (RH) for 24 hoursat room temperature, but lost moisture down to 3% while still exposed to80% RH but with the temperature increased to 160° F. A second derivativeexposed at the same conditions picked up 14% water at ambienttemperature but when heated to 160° F. while still at 80% RH lost waterdown to 1%. On both samples the cycle was repeated several times withalmost identical gain and loss of moisture due obviously only to changesin temperature.

This invention makes solar refrigeration truly solar. No hightemperature gas drier is needed to regenerate the bed because thedesiccant of this invention will obviously dry at temperatures easilyavailable in the day time in summers to the peak of the roof which mayreach temperatures as high as 160° F. to 180° F., depending on thetemperatures of the air entering the tubes at the bottom and on itsrelative humidity. The lower the initial relative humidity the lowerwill be the rise in temperature of air reaching the plenum at the peakof the roof.

The hot-dry air coming to the peak of the roof from the solar collectorson the roof not facing the sun (or at night) will pass into a heatexchanger (not depicted in the drawing) whose purpose will be toexchange heat with ambient air to cool down the hot air as much aspossible. The hot-dry air should cool down to within 5° to 10° F. ofambient air. This may in some cases require baffles on the inside of theheat exchanger and radiation fins on the outside, and such apparatus isnot very difficult to fabricate.

Once the dry air is cooled to within a few degrees above ambient it issucked into a blower which forces it to go thru moistened pads tohumidify it and to cause cooling of the air. Again the temperature thatthe humidified air will drop to will depend on how much water remains inthe air and its temperature before entering the humidification chamber.Usually the air temperature will drop to 60° to 65° F. This cool airwill generally be forced either into the house if cooling is required orinto some kind of energy storage chamber such as stones or gravel. Thisenergy storage material may be cooled down at night and thru the day thedesiccant could be dried by the sun. Cold air may be pulled from thestorage chamber whenever refrigeration is required. On the other hand,drying of the air can be done during part of the day when the roof is inthe shade and on the other part of the roof the desiccant could be driedsimultaneously by the sun, which would require less desiccant.

It is apparent that certain variations can be made without departing thespirit and scope of the present invention. In its essence, the presentinvention is based on the discovery of unique solid methyl cellulosedesiccants which can absorb large quantities of water at temperaturesbelow a syneresis temperature ranging from about 100° F. to about 195°F., preferably about 140° F. to about 160° F., depending upon itscomposition. Below this temperature it will hold the water tightly, andabove this temperature it will reject the water so that it can be easilydried by application of low temperature heat. In particular, thedesiccant can be used for drying air below the syneresis temperature;and by virtue of its nature the desiccant will undergo a solid statetransition when the temperature exceeds the syneresis temperature whichcauses it to lose its affinity for water almost completely such that onthe application of low temperature heat, as when the sun is shining andthe temperature reaches syneresis temperature, the water is quicklyswept away by an air stream, drained off or decanted from the desiccant.This unique desiccant can provide large savings in both energy andcapital costs, and is particularly useful for solar heating, andrefrigeration applications.

Having described the invention, what is claimed is:
 1. A process for thedehydration of a fluid stream with a desiccant, and regeneration of saiddesiccant which comprises contacting said fluid stream with a desiccantat a temperature below its syneresis temperature, the syneresistemperature of said desiccant ranging from about 100° F. to about 195°F., said desiccant being characterized as solid methyl cellulose whereinfrom about 0.6 to about 1.8 of the available hydroxyl groups of theanhydroglucose units constituting the basic cellulose entity have beenreplaced by methyl and other hydroxy alkyl or carboxy alkyl groupscontaining from 2 to about 4 carbon atoms where at least one-half of thesubstituting groups are methyl, and heating said desiccant above thesyneresis temperature, said solid methyl cellulose desiccant having thecapacity to lose its affinity for water, and to release water whenheated above its syneresis temperature.
 2. The process of claim 1wherein the desiccant is solid methyl cellulose wherein at leasttwo-thirds of the substituting groups are methyl, where all of thesubstituting groups are not methyl.
 3. The process of claim 1 whereinthe desiccant is solid methyl cellulose which contains carboxyl methylgroups.
 4. The process of claim 1 wherein the desiccant is solid methylcellulose and essentially all of the substituting groups are methyl. 5.The process of claim 1 wherein the syneresis temperature of the solidmethyl cellulose desiccant ranges from about 140° F. to about 160° F. 6.A process for drying air with a desiccant, and regeneration of saiddesiccant which comprises contacting said fluid stream with a desiccantat a temperature below its syneresis temperature, the syneresistemperature of said desiccant ranging from about 100° F. to about 195°F., said desiccant being a solid methyl cellulose wherein from about 0.6to about 1.8 of the available hydroxyl groups of the anhydroglucose unitconstituting the basic cellulose entity have been replaced by methyl, ormethyl and other hydroxy alkyl or carboxy alkyl groups containing from 2to about 4 carbon atoms where at least one-half of the substitutinggroups are methyl, and heating said desiccant above the syneresistemperature, said solid methyl cellulose desiccant having the capacityto lose its affinity for water, and to release water when heated aboveits syneresis temperature.
 7. The process of claim 6 wherein thedesiccant is solid methyl cellulose which contains at least two-thirdsof the substituting groups as methyl, where all of the substitutinggroup are not methyl.
 8. The process of claim 6 wherein the desiccant issolid methyl cellulose which contains carboxy methyl groups.
 9. Theprocess of claim 6 wherein the desiccant is solid methyl cellulose andessentially all of the substituting groups are methyl.
 10. The processof claim 6 wherein the syneresis temperature of the solid methylcellulose desiccant ranges from about 140° F. to about 160° F.